Placer Gold, Gold Mines & Gold Deposits Utah & Colorado

Gold is the fourth most important mineral commodity produced from the Tintic mining district after silver, lead, and zinc. Some of this gold is produced as a byproduct or coproduct of the argentiferous lead and zinc ores, but large amounts also have been produced from gold-bearing fissure veins and lodes in massive quartzite, and from large baritic jasperoid replacement ore bodies in carbonate rocks. Since its discovery in 1869 the Tintic district has produced more than 2.77 million ounces of gold, and the two most recently operating mines in the district-the Mammoth mine in the Main Tintic subdistrict and the Trixie mine in the East Tintic subdistrict-have produced predominantly gold-bearing fluxing ores. Much potentially gold-bearing terrane may remain to be explored and developed in the Tintic district, particularly in the East Tintic subdistrict.

Index page map from book showing location of the Tintic mining district, Utah,
and some other significant mining districts in Utah and Nevada

Introduction:

The Tintic mining district of west-central Utah, including its East Tintic subdistrict, is world renowned for its production of silver and lead. This district, however, also has produced important quantities of other metals, including zinc, gold, copper, cadmium, and bismuth. The gold-bearing ores of the district include not only polymetallic base metal ores from which gold is recovered as a byproduct or coproduct, but also replacement ore shoots and quartz veins and lodes that were mined principally for their content of gold. Undeveloped bodies of gold ore are known in the district, and much exploration for gold and silver ore bodies has been undertaken during the past two decades.

Generalized geology page map from book showing northern two-thirds
of the Tintic mining district

The Tintic district is in the East Tintic Mountains near the east-central boundary of the Great Basin. It is about 95 km southwest of Salt Lake City, Utah, and is traversed by U.S. Highway 6-50 and served by the Union Pacific and Denver and Rio Grande Railroads. The district was discovered in 1869, and intermittent new discoveries of concealed ore bodies have kept the district almost continuously productive to the present time. Through 1987 Tintic has produced somewhat more than 19.1 million tons of ore containing approximately 272 million oz of silver, 1.14 million tons of lead, 225,000 tons of zinc, 127,000 tons of copper, and 2.77 million oz of gold. Reserves of gold-bearing ores in active and inactive mines either are not available for publication or are unknown, but reserves are believed by the author to be small to moderate. Gold resources in unexplored or only partly explored terranes may exceed 650,000 oz, about one-fourth of past production, but they probably do not exceed 2 million oz, about two-thirds of the total historic production.

Generalized geology page map from book ofnorthern two-thirds
of the Tintic mining district showing mine locations and names, vertical
projection of known ore bodies and mineralized areas

The primary ores of the Tintic district contain galena, sphalerite, acanthite, argentite, tetrahedrite-tennantite, enargite-famatinite, proustite, hessite, calaverite, native gold, native silver, and a wide variety of relatively uncommon copper-, lead-, silver-, and bismuth-bearing sulfosalt minerals. Deep oxidation of these ores has further produced a great variety of sulfates, carbonates, silicates, arsenates, antimonates, and manganates. Some of these oxidized ores also contain native gold. Although some of the outcropping ores were rich in native gold, their limited distribution and shallow depth of erosion precluded the formation of significant placer deposits.

The Sneffels-Telluride and Camp Bird mining districts have produced base and precious metals valued at more than $300 million at the time of production. The production included about 6,200,000 ounces of gold. The ores were mined from veins and replacement deposits developed on the northwest side of the Tertiary Silverton caldera. West- dipping Paleozoic and Mesozoic sedimentary rocks rest unconformably on Precambrian quartzite and are unconformably overlain by the Tertiary Telluride Conglomerate, an important host of base metal replacement deposits. Overlying the Telluride Conglomerate are more than 1,000 meters of volcanic flows, breccias, and tuffs that are the major hosts for most of the precious metal deposits in the district. Throughout the entire district the volcanic rocks have been propylitically altered. Near the veins sericitic, argillic, and calc-silicate altered zones are extensively developed.

The district is dominated by a northwest-trending swarm of dikes, fissures, and veins radiating from the Silverton caldera. Many of the faults have been mineralized and commonly contain both veins and dikes. The veins may be divided into a southern group that has produced base and precious metals and a northern group that has produced predominantly precious metals. Age data indicate that the northwest-trending fracture system formed during caldera collapse about 25 Ma and that it was initially barren. Mineralization of the fracture system occurred later during at least two stages, at 17 Ma and 13 Ma. Fluid-inclusion studies of the veins in the district indicate filling temperatures of about 280-290 °C and very low salinities. Important controls of ore deposition were types of wall rock, vein intersections, character of fault movements, presence of intersecting dikes and faults, and abrupt changes in strike or dip. The suggested hypothesis for the origin of the ore deposits is that the metals (copper, zinc, gold, and silver, and perhaps some of the lead) were derived directly from magmatic sources and mixed with a large volume of meteoric water circulating within caldera-related fracture systems. Some of the lead was derived from nearby wall rocks traversed by the meteoric waters. This ore-forming process may have been repeated at least once within the district.

Page Map from book of Principal veins, showing names, and structures on the
northwest side of the silverton caldera, Sneffels-Yelluride and
Camp Bird mining districts, Colorado

Introduction:

The Sneffels-Telluride and Camp Bird mining districts are located in the western San Juan Mountains, Colorado. For purposes of this report, the two districts will be treated as one and will be referred to simply as the "district." Both districts were developed within the extensive radial vein-dike-fault system on the northwest side of the Silverton caldera.

Approximately 22 million tons of ore exceeding $300 million in value at the time of production has been mined from the district. Of this about 1.8 million tons of ore containing 0.8 oz Au/ton, 2.5 oz Ag/ton, and 2-3 percent Co, Ph, and Zn combined, has been produced from the Camp Bird vein (U.S. Bureau of Mines records). Early production (pre-1900 to 1950) from these radial veins was of the precious metals, and the ores averaged about 0.3 oz Au/ton and 3.5 oz Ag/ton. Parts of some lodes were much richer; for example, about 800,000 tons of ore from the Camp Bird vein contained 1.0 oz Au/ton and 2 oz Ag/ton; approximately 100,090 tons of ore from the Virginius vein contained 0.9 oz Au/ton and more than 100 oz Ag/ton; and about 5,000 tons of ore from the Mt. Top mine contained 0.1 oz Au/ton and more than 200 oz Ag/ton (U.S. Bureau of Mines records). Later production (1950-1975) was from the deeper parts of the veins where the ores typically contained 0.07 oz Au/ton and 1.95 oz Ag/ton and approximately 2.33 percent Pb, 0.72 percent Cu, and 3.62 percent Zn. Total amount of gold produced from the district is about 6,200,000 oz.

Prospecting in the San Juan region commenced in the late 1860's, and in 1870 the district was prospected by R.C. Darling. Indian hostilities prevented extensive exploration, and few claims were filed until a new treaty with the Ute Tribe was negotiated in 1874, after which prospecting flourished. The original discovery on the Camp Bird property was made in 1877: early interest was directed at silver mining, but in 1896, Thomas Walsh purchased most of the claims in the Camp Bird group and developed the property into a highly successful gold mine. Since the turn of the century, mining in the district has waxed and waned following this pattern: (1) discovery of promising ore in several structures and the attempt to produce from numerous individual mines, (2) consolidation of properties and more efficient mining leading to successful production, until declining grade and increasing costs ( and (or) fluctuating metal prices) ended operations, (3) new discoveries and a repeat of (1) and (2) above.

Deposits similar to those in the district occur elsewhere in the San Juan Mountains at Silverton, Creede, and Sunnyside. Deposits at the Mayflower mine in Utah are similar to those in the Argentine-Black Bear vein system in the district; all are base and precious metal deposits that formed in extensive structural systems exhibiting similar ores, temperatures of formation, salinity of fluids, and wall rock alteration. In a broad sense the ore deposits in the district may be compared to those in volcanic areas in Japan
 

Abstract:

The Alma district, discovered in 1860, has produced more than 1.3 million ounces of gold and substantial silver, lead, zinc and copper. The district lies on the east flank of the north-trending Sawatch uplift. Precambrian gneisses, schists, and granites, and overlying Paleozoic marine sedimentary rocks have been intruded by Laramide intermediate to silicic porphyry dikes, sills, and a stock. The porphyries were strongly altered, mostly before ore deposition. Major north-northwesterly reverse faults with associated drag folds, such as the prominent London fault, were major controls on localization of ore deposits.

Three principal types of gold ores formed following emplacement of the porphyries. (1) Gold-bearing quartz veins produced most of the value of the Alma district. The greatest vein production came from the "London ore zone," in shattered porphyry sills beneath west-dipping shale on the limb of a drag syncline adjacent to the London fault. The veins contain pyrite, dark-colored sphalerite, galena, chalcopyrite, and free gold. During early mining, gold grades averaged 1.86 oz Au/ton. (2) Next in importance are silver-lead replacement mantos formed in dolomitic limestone host rocks adjacent to fissures. The replacement deposits are characterized by a gangue of ferroan dolomite, barite, jasperoid, and quartz, and by ore minerals including light-colored sphalerite, galena, pyrite, chalcopyrite, and locally tennantite, luzonite, argentite, freibergite, tetrahedrite, covellite, and chalcocite. The replacement deposits are somewhat similar to mantos in the nearby Leadville district. Numerous secondary minerals formed from the manto ores in the weathered zone. Ores mined from the replacement deposits contained 0.04-0.26 oz Au/ton. (3) The third ore type is gold deposits in quartzite; the deposits consist of pyrite-sphalerite plenfrchalcopyrite replacements of certain beds adjacent to Minor fds, and of small veins. Gold content of mined quartzite ores was 0.05-0.30 oz Au/ton.

District-scale mineral zoning is poorly developed, but some smaller clusters of deposits are zoned. Highest gold values along the London fault are near the center of the district. Groups of silver-lead replacement deposits are rich in silver centrally, and rich in barite peripherally. Some gold and pyrite-bearing quartz veins in Precambrian rocks grade upward into silver-Iead-manganese veins, which in turn merge upward into silver-lead replacement ore bodies in overlying Paleozoic carbonate rocks. The principal ores of the district probably formed about 35 Ma from heated solutions driven by porphyry intrusion. The Alma district appears to have high potential for new gold discoveries.

Page map from book of Colorado showing location of the
Alma district andother major gold producing districts.

Introduction:

The Alma mining district is in the central part of the Colorado mineral belt, about 105 km southwest of Denver and 10 km east of Leadville. It lies in Park County at the northwest edge of South Park, on the east flank of the Mosquito Range. Gold was discovered at the Phillips lode and other nearby deposits in the upper reaches of Buckskin Gulch in the headwaters of the middle fork of the South Platte River in 1860. By 1863 mining had exhausted the oxidized ores near the surface, and deeper sulfide ores, some of large volume as at the Phillips mine, could not be treated successfully and were abandoned. Silver ore was discovered on Mount Lincoln and Mount Bross, northwest of the town of Alma, in 1871, resulting in substantial silverr production that continued until 1892 when the price of silver dropped. Silver ore was discovered in the London vein west of Alma in 1873, and silver and gold production from these deposits was almost continuous until 1942. Gold placer deposits along the South Platte River east of Alma were first mined in the 1870's, although greatest production occurred during the period 1904-1942. OnIy small and sporadic gold production has occurred in the Alma district following World War II. Cobb and Boulder Gold announced the start of new production in the London mine in May 1988.

Production of gold from the Alma district through 1959 amounted to about 1,350,000 OZ, of which about 1,320,000 oz came from lode deposits. In the period 1859-1951 the district also produced 7,636,045 oz of silver, 58,892,775 Ibs of lead, 3,103,485Ibs of copper, and 8,263,500 Ibs of zinc. Zinc recovery did not begin until 1937, and consistent zinc recovery started in 1940. Minable reserves in the London mine of 400,000 tons of gold ore averaging about 0.4 oz Au/ton (12.3 g Au/ton) were reported in May 1988.

Page map from book showing Generalized geology of
the Alma district, showinglocations of principal mines

Ore Deposits:

Ore deposits in the Alma district are part of the northeast -trending Colorado mineral belt. Within the district they form clusters that appear to be related to major structures. For example, near London Mountain, the most productive mines of the district lie in a narrow zone in the footwall of the London fault; farther south the Sherwood and nearby mines are in the hanging wall of the London fault; about 3 km west of Alma a north-trending zone of mines is in the hanging wall of the Cooper Gulch fault; and mines farther north near Mount Lincoln and Mount Bross lie on a structural terrace that  is part of the same "tectonic unit" as the Cooper Gulch fault. Despite the fact that major faults exercise rough structural control of ore distribution, individual ore bodies have been localized exclusively on minor structures, some of which are en echelon zones of" minor high-angle fissures.

The most important deposits fall into three classes: (1) gold-bearing quartz veins in or against porphyry sills near the base of the Minturn Formation, (2) silver-lead replacement deposits in carbonate formations, and (3) gold deposits in quartzite of the Sawatch Quartzite. Quartz-pyrite-molybdenite veins in Precambrian rocks just northwest of the Buckskin stock have not been mined, and a vuggy quartz-tetrahedrite-fluorite-rhodochrosite vein in Precambrian rocks at the Sweet Home mine southeast of the Buckskin stock has been mined mostly for mineral specimens.

Abstract:

The leadville district has yielded 3.25 million ounces of gold and 256 million ounces of silver in 127 years of production since 1860. Placer gold, quartz-pyrite-precious metal veins, and base- and precious-metal, dolomite-hosted replacement bodies (of leadville type mineralization) have contributed the bulk of the precious metals in the district. Ore bodies of leadville type mineralization have been the major producer of gold and silver.

Gold occurs principally as electru m (Au:Ag = 1 : 1) whereas silver is present also in argentian tetrahedrite. Both minerals are late in the sulfide paragenetic sequence. Electrum occurs either in a peripheral zone adjacent to a central base metal zone within the replacement bodies or within and adjacent to veins that cut the replacement bodies. Argentian tetrahedrite occurs within the base metal zone, particularly where late veinlets cross that zone. District-wide, gold is concentrated in a zone peripheral to and within the centrally located Breece Hill stock, and along low-angle, east-dipping thrust faults west of Breece Hill. Silver is concentrated in a zone peripheral to the gold-enriched zone.

The precious-base metal deposits are related to a middle Tertiary igneous intrusive as shown by metal zoning about the Breece Hill stock. Sulfur isotope analyses on pyrite (8 S4 S: + 1.2 to + 3.2 per mil), sphalerite (8 S4 S: -D.5 to + 2.2 per mil), and galena (8 S4 S: -2.4 to +0.7 per mil) yield calculated temperatures of crystallization from pyrite-sphalerite and sphalerite-galena pairs of 420:!:21 °C and 450=/-23 °C, respectively. These temperatures agree closely with pressure-corrected fluid-inclusion values during sulfide precipitation of 380-445 °C. The ore fluid 8S4SHaS is estimated to have been + 1.9 per mil and magmatic in origin. The age of mineralization in the leadville district has been determined by fission-track dating of annealed apatite and zircon from pre-ore igneous rocks to be 33.8:1: 5.0 Ma.

Index page map from book of Colorado mineral belt showing location of
the Leadville district with principal mountain ranges outlined

The Leadville district was discovered in 1860 by prospectors exploring for gold in the Arkansas River and its tributaries. Despite extensive Pleistocene glaciation of the Mosquito Range, one tributary, California Gulch, had not been scoured of its heavy-mineral content. Subsequent gold placer operations upstream recovered significant quantities of cerussite with the gold. Ultimately, the source of the silver-rich lead carbonate was found in surface and near-surface exposures of replacement bodies within the Paleozoic carbonate rocks. As a result of these discoveries, the district has been a metal producer for about 130 years. The precious metal content, particularly silver, was very important in the first 40 years of production; however, since the deregulation of gold pricing, the importance of gold in district ore bodies has been reestablished. Estimated precious metal production from 1860 to 1986 is 3.25 million ounces of gold and 256 million ounces of silver.

The Leadville district is near the central part of the Colorado mineral belt, a northeast-striking zone of Laramide to Tertiary igneous activity and associated mineral deposits. The igneous activity appears to have been localized by a series of shear zones that have been active since Precambrian time. Reported here will be the aspects that pertain to precious metal distributions in the district. It is not the intent of this report to discuss regional and local stratigraphy and structure; they have been reported else where as will be indicated herein. Ore mineralogy will be reported only in the detail required to discuss gold-silver distributions, and stable isotope data will be presented in a preliminary fashion.

Geologic page map from book showing the
Leadville district, Lake County, Colorado

Geologic page map from book of  the Leadville district showing the Breece Hill
intrusive center, distribution of ore bodies, and district-wide gold and silver zonation.
Long-dashed lines separate areas of the indicated gold content and Ag:Au ratios of ores

Mineral deposits at Leadville are of seven types: 1. Placers in California Gulch; 2. Contact metamorphic magnetite-calc-silicate carbonate bodies adjacent to two stocks; 3. Vein quartz-pyrite-precious metal-(tungsten) deposits within and adjacent to the Breece Hill stock; 4. Veinlet and disseminated quartz-pyrite-gold in porphyry; 5. Base metal veins; 6. Base- and precious-metal dolomite-hosted replacement bodies (referred to here as of "Leadville type mineralization" (LTM»; and 7. Silver-barite-minor base-metal, dolomite- hosted replacement bodies (referred to here as of "Sherman type mineralization" (STM». All but the fourth type have contributed to district production. In this report the emphasis will be on those with precious metal contents, that is, types 3, 6, and 7. Little scientific work has been directed to the district placers, and only traces of unmined placer material remain within California Gulch.

Diagrammatic page map from book showing district-wide
cross sectionof ore-body types

About The Book: Gold-Bearing Polymetallic Veins and Replacement Deposits - Part II, Bulletin 1857F, first edition published 1990, by Hal T. Morris, Fredrick S. Fisher, Daniel R. Shawe, Tommy B. Thompson (U.S.G.S)